diff --git a/internal/backend/remote-state/s3/backend.go b/internal/backend/remote-state/s3/backend.go
index 15e25cd2e8..d826dbaf1b 100644
--- a/internal/backend/remote-state/s3/backend.go
+++ b/internal/backend/remote-state/s3/backend.go
@@ -417,15 +417,22 @@ var assumeRoleSchema = singleNestedAttribute{
 			},
 		},
 
-		// NOT SUPPORTED by `aws-sdk-go-base/v1`
-		// "source_identity": stringAttribute{
-		// 	configschema.Attribute{
-		// 		Type:         cty.String,
-		// 		Optional:     true,
-		// 		Description:  "Source identity specified by the principal assuming the role.",
-		// 		ValidateFunc: validAssumeRoleSourceIdentity,
-		// 	},
-		// },
+		"source_identity": stringAttribute{
+			configschema.Attribute{
+				Type:        cty.String,
+				Optional:    true,
+				Description: "Source identity specified by the principal assuming the role.",
+			},
+			validateString{
+				Validators: []stringValidator{
+					validateStringLenBetween(2, 64),
+					validateStringMatches(
+						regexp.MustCompile(`^[\w+=,.@\-]*$`),
+						`Value can only contain letters, numbers, or the following characters: =,.@-`,
+					),
+				},
+			},
+		},
 
 		"tags": mapAttribute{
 			configschema.Attribute{
@@ -1077,6 +1084,9 @@ func (b *Backend) Configure(obj cty.Value) tfdiags.Diagnostics {
 		if val, ok := stringAttrOk(assumeRole, "session_name"); ok {
 			ar.SessionName = val
 		}
+		if val, ok := stringAttrOk(assumeRole, "source_identity"); ok {
+			ar.SourceIdentity = val
+		}
 		if val, ok := stringMapAttrOk(assumeRole, "tags"); ok {
 			ar.Tags = val
 		}
diff --git a/internal/backend/remote-state/s3/backend_complete_test.go b/internal/backend/remote-state/s3/backend_complete_test.go
index a8d1ec342c..ce513a1b87 100644
--- a/internal/backend/remote-state/s3/backend_complete_test.go
+++ b/internal/backend/remote-state/s3/backend_complete_test.go
@@ -1081,23 +1081,6 @@ aws_secret_access_key = DefaultSharedCredentialsSecretKey
 			),
 		},
 
-		// NOT SUPPORTED: AssumeRoleSourceIdentity
-		// WAS: "config AssumeRoleSourceIdentity"
-		// "with source identity": {
-		// 	config: map[string]any{
-		// 		"access_key":                  servicemocks.MockStaticAccessKey,
-		// 		"secret_key":                  servicemocks.MockStaticSecretKey,
-		// 		"role_arn":                    servicemocks.MockStsAssumeRoleArn,
-		// 		"session_name":                servicemocks.MockStsAssumeRoleSessionName,
-		// 		"assume_role_source_identity": servicemocks.MockStsAssumeRoleSourceIdentity,
-		// 	},
-		// 	ExpectedCredentialsValue: mockdata.MockStsAssumeRoleCredentials,
-		// 	MockStsEndpoints: []*servicemocks.MockEndpoint{
-		// 		servicemocks.MockStsAssumeRoleValidEndpointWithOptions(map[string]string{"SourceIdentity": servicemocks.MockStsAssumeRoleSourceIdentity}),
-		// 		servicemocks.MockStsGetCallerIdentityValidEndpoint,
-		// 	},
-		// },
-
 		// WAS: "assume role error"
 		"error": {
 			config: map[string]any{
@@ -1524,24 +1507,23 @@ aws_secret_access_key = DefaultSharedCredentialsSecretKey
 			},
 		},
 
-		// NOT SUPPORTED: AssumeRoleSourceIdentity
 		// WAS: "config AssumeRoleSourceIdentity"
-		// "with source identity": {
-		// 	config: map[string]any{
-		// 		"access_key": servicemocks.MockStaticAccessKey,
-		// 		"secret_key": servicemocks.MockStaticSecretKey,
-		// 		"assume_role": map[string]any{
-		// 			"role_arn":        servicemocks.MockStsAssumeRoleArn,
-		// 			"session_name":    servicemocks.MockStsAssumeRoleSessionName,
-		// 			"source_identity": servicemocks.MockStsAssumeRoleSourceIdentity,
-		// 		},
-		// 	},
-		// 	ExpectedCredentialsValue: mockdata.MockStsAssumeRoleCredentials,
-		// 	MockStsEndpoints: []*servicemocks.MockEndpoint{
-		// 		servicemocks.MockStsAssumeRoleValidEndpointWithOptions(map[string]string{"SourceIdentity": servicemocks.MockStsAssumeRoleSourceIdentity}),
-		// 		servicemocks.MockStsGetCallerIdentityValidEndpoint,
-		// 	},
-		// },
+		"with source identity": {
+			config: map[string]any{
+				"access_key": servicemocks.MockStaticAccessKey,
+				"secret_key": servicemocks.MockStaticSecretKey,
+				"assume_role": map[string]any{
+					"role_arn":        servicemocks.MockStsAssumeRoleArn,
+					"session_name":    servicemocks.MockStsAssumeRoleSessionName,
+					"source_identity": servicemocks.MockStsAssumeRoleSourceIdentity,
+				},
+			},
+			ExpectedCredentialsValue: mockdata.MockStsAssumeRoleCredentials,
+			MockStsEndpoints: []*servicemocks.MockEndpoint{
+				servicemocks.MockStsAssumeRoleValidEndpointWithOptions(map[string]string{"SourceIdentity": servicemocks.MockStsAssumeRoleSourceIdentity}),
+				servicemocks.MockStsGetCallerIdentityValidEndpoint,
+			},
+		},
 
 		// WAS: "assume role error"
 		"error": {
diff --git a/internal/backend/remote-state/s3/backend_test.go b/internal/backend/remote-state/s3/backend_test.go
index 31536bbce5..1f0df00f82 100644
--- a/internal/backend/remote-state/s3/backend_test.go
+++ b/internal/backend/remote-state/s3/backend_test.go
@@ -2001,8 +2001,12 @@ func TestAssumeRole_PrepareConfigValidation(t *testing.T) {
 			},
 		},
 
-		// NOT SUPPORTED by `aws-sdk-go-base/v1`
-		// "source_identity"
+		"source_identity": {
+			config: map[string]cty.Value{
+				"role_arn":        cty.StringVal("arn:aws:iam::123456789012:role/testrole"),
+				"source_identity": cty.StringVal("source-identity"),
+			},
+		},
 
 		"with tags": {
 			config: map[string]cty.Value{